1. Field of the Invention
This invention relates to an incremental measuring system, particularly for measuring lengths, comprising a scale member, which comprises at least one track provided with a measuring scale and is carried at one side edge by and accommodated in a tubular protective carrying housing, which has a slot, through which at least one actuating member extends, and a scanning unit, which is guided to be movable parallel to the scale member and is provided with a common light source, which is operable to illuminate photoelectric receivers with collimated light that is transmitted via an optical system and through the scale member and the measuring scale and through scanning gratings associated with respective ones of the receivers.
This invention relates also to a process for trimming an incremental measuring system which comprises a scale member that is provided with a measuring scale and which also comprises a scanning unit which is guided to be movable parallel to the scale member and is provided with a common light source, which is operable to illuminate photoelectric receivers through an optical system which comprises a reflector for deflecting light emitted by the light source and condensing means, and through the measuring scale and through scanning gratings associated with the photoelectric receivers so that the latter are illuminated with light which is modulated in response to the movement of the scanning unit relative to the measuring scale.
Such a measuring system may also be used as an angular position detector if a circular scale and a corresponding circular protective housing are employed.
2. Description of the Prior Art
In such measuring systems the measuring scale of the scale member is scanned through the scanning gratings with transmitted light. The use of a single light source affords the advantage that the power requirement is low and that the measuring field will be illuminated rather uniformly. On the other hand, the light path between the light source and the condenser involves a considerable space requirement so that the tubular protective and carrying housing must have a large inside width. Whereas it is known to transmit the light through optical fibers, they will add to the cost of the entire system and involve considerable light losses. In measuring systems in which scanning is effected with reflected light and a scale strip is used, the scanning unit is provided at the top of the protective housing and the scale strip is supported by said housing top at its center, it is known that the light source of the illuminating means is arranged in the scanning unit at a longitudinal distance from the location at which the scale is currently scanned and the cone of light emitted by the light source is deflected by a mirror. Such an arrangement will involve a large overall length of the scanning unit and problems will arise in connection with the guiding of the scanning unit and its coupling to an actuating member for displacing the scanning unit. As has been mentioned that arrangement can be used only if the measuring strip has a reflective measuring scale and extends horizontally at the top of the tubular housing. In known arrangements having a single light source, difficulties have been involved in the adjustment of the photoelectric receivers. Photoelectric receivers often vary substantially in their response so that their output signals may differ by as much as 15% in case of the same illumination. In the previous practice such differences between the responses of several receivers have been compensated by circuits which include adjustable potentiometers and succeed respective receivers so that the d.c. components otherwise occurring in the signals will be suppressed if the receivers are arranged in back-to-back connections in the usual manner. In addition to differences in the signal amplitude, another error which may occur is the so-called phase error, which resides in that the phase displacement of the signals which are produced by the receivers in response to the light received through the scanning gratings are not equal to the desired phase displacements usually amounting to 90 and 180 degrees, respectively, but differ from said desired values. For the compensation of that phase error it is also usual to provide compensating circuits which succeed the receivers and which are expensive and just as potentiometers tend to age in the course of time. Besides, the total illumination will also change as a result of the performance of the light source or of the optical transmittance of the light-transmitting media. In simple measuring systems it is hardly possible to ensure that the output signals will have a desired waveform, such as a pure sine shape or triangular shape. This can be accomplished only by means of expensive circuitry, particularly by means of a microprocessor, and this will also involve a high expenditure for the circuitry and the adjustment which is required.
It is known, e.g., from Published German Application 23 49 944 that the scale member extends with an inclination, particularly of 45.degree., into the cavity of the carrying housing from that side of said housing which is opposite to the slot. In that case the entire scanning unit or only the scanning plate which contains the scanning gratings is guided directly on the scale member, which protrudes into the cavity, and a backlashfree coupling is desired between the scanning unit and the actuating member which is connected to the pickup provided on the machine tool or the like. In said known arrangement the scanning plate and the photoelectric receivers are guided on the underside of the scale member and a separate light source consisting of a light-emitting diode is provided for each receiver and is held by a separate carrier on the other side of the scale member. In that case each light-emitting diode can be adjusted to vary its spacing from the scale member so that the illumination of the associated photodetector is individually altered. But that arrangement can only be provided if large distances are provided between the several scanning gratings, receivers and illuminating means so that a large overall space requirement, a large luminous power and an expensive structure will be required. In systems comprising photodetectors associated by respective light sources it is also known to provide the light sources consisting, e.g., of light-emitting diodes, in perforated plates and to adjust the actual light output by means of screws or the like which protrude into the apertures in the plate. But in that case the compensation will be inaccurate and will require an expensive adjustment and a correspondingly bulky structure will be required.